This invention relates to a top driven drilling machine for use in a derrick and a method for preventing damage to the threaded ends of drill pipe. More particularly, the invention provides for lowering the pipe and contacting a drill string for connection without damaging impact. The invention also includes pulling the pipe away from the drill string and preventing rebound impact when removing the drill string from a well.
Conventional rotary drilling requires the use of a rotary table, a motor mounted on or below the rig floor for rotating the table, and a kelly for rotationally connecting the table to the drill string. In recent years, these drilling units are being replaced by or retrofitted with top driven drilling systems which rotate the drill string by a motor suspended within a standard derrick or mast from a traveling block.
Hereafter, these top driven machines will be referred to as a power swivel. The drilling motor is connected to the drill string by a cylindrical stem or sub assembly extending downwardly within the derrick from the drill motor. Drilling is accomplished by the powered rotation of the drill string. A cutting tool or bit is placed at the bottom end of the drill string which, through the rotational energy supplied by the drill motor, cuts through the earth's formations and deepens a well. As the well is drilled, the bit becomes worn and must be replaced periodically. When replacement of the bit becomes necessary, if the well needs to be surveyed, or the well needs to be lined with casing, a portion of the drill string corresponding in length to one or more sections of drill pipe must be removed from the well and pulled above the rig floor. This portion of the drill string is removed and stored on the rig. The drill string is again pulled from the well exposing the next section above the floor and is similarly removed. This sequence, usually referred to as tripping out, is continued until the entire drill string is removed from the well. After the bit is replaced, the surveying is completed or the casing set, the drill string is reassembled; i.e. tripping in, by connecting all the pipe sections previously removed.
For drilling equipment utilizing a power swivel, the pipe handling operation is controlled remotely from a console on the derrick platform. Because the drilling unit is large and somewhat cumbersome to handle, the threads on the sub assembly or pipe ends frequently become damaged when connecting or disconnecting the individual pipe stands from the sub assembly of the power swivel or the drill string. When their threads become damaged, the sub assembly or pipe must be temporarily taken out of service until the threads can be remachined to serviceable condition. This increases the costs of drilling because of delay time, reconditioning costs, and the increased inventory of drill pipe required. Furthermore, pipe having damaged thread and not taken out of service for repair could result in washout of the threads as well as a separation of the drill string inside the well bore.
A stand of drill pipe may include as many as three sections of pipe. When a drill string must be removed from a well, a traveling block vertically lifts the drill string from the well a distance above the floor of the drilling rig corresponding to a length of the stand of pipe. The weight of the drill string is then supported by wedges or slips located at the floor of the drilling rig. The top stand of drill pipe is removed from the drill string normally by first disconnecting the sub assembly from the top end of the pipe. A wrench assembly or tongs is used to hold and prevent rotation of the dril pipe when rotating or "spinning out" the sub assembly from the top end of the drill pipe. As the sub assembly becomes threadably disengaged from the drill pipe, the upward tension of the power swivel causes the sub assembly to be pulled away from the drill pipe. However, the sub assembly could impact against the top end of the drill pipe if there is a rebound after disengagement. Similar impact may occur when reassembling drill pipe to the drill string and when connecting the sub assembly to the top end of the drill pipe. Since the point of connecting the sub assembly to the drill pipe is well above the floor of the drilling rig, the operator's line of sight may be obscured. Furthermore, the control system for remotely operating the power swivel lacks sensitivity to quickly stop the traveling block when lowering the power swivel. These impacts usually result in damaged threads or damaged sealing surfaces, either of which requires removal of the damaged item from service.
There have been many attempts over the years to provide cushioning devices to prevent thread damage when "spinning out" or "spinning in". However, these devices do not provide cushioning for both operations. More importantly, these cushioning devices have not completely eliminated impacts because thread damage still occurs. U.S. Pat. No. 2,712,932 discloses a telescoping cushioning device including a helical compression spring positioned within a cylinder. The cylinder is positioned at the top end of a stand of pipe to be connected to a drill string. If the stand of pipe impacts the drill string when lowered, the weight of the pipe compresses the spring. U.S. Pat. No. 3,766,991 discloses a power swivel utilizing shock absorbers enabling smooth handling of pipe. The hangers for the swivel are formed by hydraulic cylinders. The upward movement of the power swivel resulting when the pipe section is unthreaded is compensated for by the downward movement of a piston under the cushioning pressure of a fluid. This assures smooth release of the threads without sudden upward jumping of the power swivel when the last thread is released. U.S. Pat. Nos. 3,838,613 and 4,251,176 disclose the use of counterbalance valves for weight compensation when adding or removing pipe from a drill string.
Nevertheless, the above described passive cushioning or counterbalance devices lack the sensitivity to completely eliminate the impact problem. Furthermore, these devices generally do not provide the same degree of thread protection during both pipe handling situations discussed above. There remains a long felt need for a device which will reduce impact and thread damage both when making and breaking connections between drill pipe and a sub assembly or between drill pipe and a dril string. Our inventin overcomes this problem by providing an active counterbalance wherein the load capacity of the counterbalance is adjusted depending on whether pipe is being removed or added to a drill string. The weight of the power swivel is known. When adding pipe to the drill string, the load capacity of the counterbalance is adjusted to slightly greater than the weight of the power swivel. Suspending the pipe from the power swivel displaces downwardly the power swivel to a full extended position. As the stand of pipe is lowered and contacts the drilling string, the power swivel is displaced upwardly to a retracted position preventing overloading of the threads. As the pipe is threaded into the drill string, the power swivel is displaced downwardly toward the extended position. The load capacity of the counterbalance is now adjusted to slightly less than the weight of the power swivel. The power swivel continues to be lowered until the subassembly contacts the upper end of the drill pipe. The sub assembly is displaced upwardly toward the retracted position preventing overloading of threads or sealing surfaces. As the sub assembly is then threaded into the drill pipe, the sub assembly is displaced downwardly toward the extend position. When a drill pipe is to be removed from a drill string, the load capacity of the counterbalance is adjusted to slightly greater than the weight of the power swivel while the counterbalance is fully extended. The drill pipe is held to prevent rotation while the sub assembly is rotated. As soon as the sub assembly is disengaged from the upper end of the drill pipe, the upward tension of the counterbalance pulls the power swivel away from the drill pipe without rebound or impact.